1. Field of the Invention
The present invention relates to a horizontal and tuning fork vibratory microgyroscope for detecting angular velocity and angular acceleration of an inertial object when the inertial object is rotated, and more particularly to a horizontal and tuning fork vibratory microgyroscope in which resonance directions of the microgyroscope are on the same horizontal plane in both sensing and driving modes, whereby its sensing characteristics are highly improved irrespective of changes to its vertical thickness determined in a process for manufacturing the microgyroscope.
2. Description of the Related Art
As well known to those skilled in the art, a gyroscope is widely used as a sensor for detecting angular velocity and angular acceleration of an inertial object when the inertial object is rotated. As such, a gyroscope may be divided into a mechanical gyroscope and a vibratory gyroscope, according to the kind of force applied to the gyroscope. The vibratory gyroscope may be sub-divided into a ceramic gyroscope and a MEMS (micro electro-mechanical system) gyroscope, according to manufacturing process of the gyroscope. The MEMS gyroscope is manufactured using a semiconductor manufacturing process. The vibratory gyroscope may also be sub-divided into a horizontal gyroscope and a vertical gyroscope, according to the direction of the force applied to the gyroscope. The horizontal gyroscope uses a Coriolis force generated in the direction parallel to the horizontal plane of the velocity of the gyroscope. On the other hand, the vertical gyroscope uses a Coriolis force generated in the direction perpendicular to the horizontal plane of the velocity of the gyroscope.
The Coriolis force used in the vibratory gyroscope can be expressed by the following equation:Fc=2m·Ω·V Where, m is mass of the inertial object, Ω is angular velocity, and V is velocity. The direction of the Coriolis force Fc is determined by the axis of the velocity V and the rotational axis of the angular velocity Ω. The vibratory gyroscope may be applied to an apparatus for detecting vibration generated by the inertial object to compensate for the detected vibration.
The vibratory gyroscope mainly comprises a stationary structure and a vibrating structure. The vibrating structure includes a driving unit and a sensing unit. The driving unit serves to resonate the vibrating structure by means of its self-oscillation to form a sensing condition in a driving mode, and the sensing unit serves to resonate the vibrating structure by means of the Coriolis force Fc applied in the direction perpendicular to the direction of the acceleration or the angular acceleration, which corresponds to the vibration of the inertial object. The direction of the resonance in the driving mode is perpendicular to the direction of the resonance in the sensing mode, and the size of a capacitor is measured on the basis of the magnitude of the Coriolis force Fc. It is required that the movement in the driving mode be large and the sensitivity in the sensing mode be excellent in order to improve the sensing performance of the vibratory gyroscope.
Systems for detecting the voltage of the gyroscope in the driving mode may be divided into a system for measuring the capacitance corresponding to the Coriolis force, which is converted into the voltage, and a rebalance system for measuring the voltage necessary to control the movement of the gyroscope caused by the Coriolis force.
The vibratory gyroscope as mentioned above is applied to an apparatus for preventing hand quiver in a video camera, an airbag for a vehicle, an unmanned airplane, and a head mount display (HMD).
As mentioned above, the gyroscope is a sensor for detecting the angular velocity of an inertial object in a predetermined direction. Consequently, it is required that the vibratory gyroscope be unaffected by any angular velocity or movement of the inertial object in directions different from the predetermined direction in which the angular velocity of the inertial object is to be measured. Sensitivity to movement in directions other than the fixed direction in which the angular velocity of the inertial object is to be measured is defined as cross talk or cross sensitivity. A sensor for measuring physical quantity has the sensitivity of which value is minimized. The sensitivity is generally limited to below a predetermined value depending upon specifications of the products.
A description of a conventional microgyroscope will now be given.
FIG. 1 is a plan view illustrating a conventional horizontal microgyroscope 10. As shown in FIG. 1, the conventional horizontal microgyroscope 10 comprises: a substrate; a vibratory structure 30 having first and second stripe portions 15 and 15′ disposed in parallel with each other, first and second combs 20 and 21 formed at one side of the first and second stripe portions 15 and 15′ respectively, and a plurality of connecting portions 16 for connecting the first and second stripe portions 15 and 15′; elastic means 11, 12 and 12′ for elastically supporting the vibratory structure 30 in such a manner that the vibratory structure 30 is spaced from the substrate by a predetermined gap; driving means 13 having a third comb 19 interposed between the first comb 20 of the first stripe portion 15 for applying the vibratory structure in one direction due to an electrostatic force; sensing means 14 having a fourth comb 21 interposed between the second comb 22 of the second stripe portion 15′ for sensing the movement of the vibratory structure 30 driven by the driving means 13 through a change of capacitance; and a plurality of sensing electrodes 18 disposed between the connecting portions 16 of the vibratory structure 30 on the same plane as that of the vibratory structure 30 to be spaced from the substrate at a predetermined gap for sensing displacement of the vibratory structure 30 due to a Coriolis force through a change of capacitance. The details of the aforesaid conventional horizontal microgyroscope are disclosed in U.S. Pat. No. 5,747,690.
As shown in FIG. 1, the conventional horizontal microgyroscope is driven in the horizontal direction X using the comb 21, and a Coriolis vibration of the vibratory structure 30 in the vertical direction is detected by means of the sensing electrode 18. Specifically, when the vibratory structure 30 is vibrated in the horizontal direction X by application of an alternating current to the combs 19, 20, 21 and 22 disposed at the both sides of the vibratory structure 30, the vibratory structure 30 is vibrated in the direction of Y by means of a Coriolis force when the angular velocity is applied in the direction of Z. The magnitude of the aforesaid vibration is proportional to the applied angular velocity. Consequently, angular velocity is measured by detecting the vibration of the vibratory structure 30 in the direction of Y in the form of a vibration frequency using the sensing electrode 18.
With the conventional horizontal microgyroscope, the vibratory structure is initially vibrated in the direction of X. When an outer angular velocity is applied to the vibratory structure, a Coriolis movement is generated, by which the vibratory structure is vibrated in the direction of Y to detect the angular velocity. Consequently, the vibrations transmitted in the directions of X and Y may have bad effects on the results of the detection when the angular velocity is detected as mentioned above. Especially, when the vibration is transmitted in the direction of Y, which is very sensitive, the effects are increased. The conventional horizontal microgyroscope has low detection performance especially when vibrating at a rate near a natural frequency among the external vibrations. Vibrations over frequency bands other than the natural frequency may be decreased by using an electric filter. According to the principle of the gyroscope, the angular velocity is modulated by the signal of the natural frequency (resonant frequency). Consequently, there exists a component which cannot be electrically offset in the gyroscope, by which the sensing performance of the gyroscope is decreased.
FIG. 2 is a plan view illustrating a conventional tuning fork microgyroscope. As shown in FIG. 2, the conventional tuning fork microgyroscope comprises: weighted element 64 and 66 suspended to rotate about a first axis and adapted to vibrate in a direction substantially orthogonal to the first axis; a pair of driven electrodes 36 and 38 projecting from the weighted elements 64 and 66 in the direction of vibration; a pair of driving electrodes 51 and 52 freely meshing with the pair of driven electrodes 36 and 38; a driving electronic circuit 71 of vibration drive contacting, through the weighted elements 64 and 66, the pair of driven electrodes 36 and 38, and the pair of driving electrodes 51 and 52 with opposite polarity signals to induce vibration of the weighted elements 64 and 66; a plurality of position sensors 41, 42, 43 and 44 placed at locations facing the weighted elements 64 and 66; a sensing electronic circuit 72 responsive to at least a subset of the plurality of position sensors 41, 42, 43 and 44. The details of the aforesaid conventional tuning fork microgyroscope are disclosed in U.S. Pat. No. 5,349,855.
The aforesaid conventional tuning fork microgyroscope uses a tuning fork vibrating mode, in which sensing is carried out in the direction perpendicular to the ground. The conventional tuning fork microgyroscope includes a structure having a horizontal direction of vibration in a driving mode and a vertical direction of vibration in a sensing mode. In the aforesaid structure, the largest output is obtained when the frequency in the horizontal direction is identical to the frequency in the vertical direction. Consequently, a tuning process for electrically tuning the resonant frequencies to be identical or similar is carried out after the tuning fork microgyroscope, is manufactured.
The resonant frequencies in the horizontal and vertical directions must be identical. However, the direction in the sensing mode is horizontal while the direction in the driving mode is vertical. Consequently, it is required that the height and thickness of elastic elements, such as springs, which vibrate in the horizontal and vertical directions, be identical, which makes tuning of the frequency very difficult. The frequency in the horizontal direction is sensitive to an etching process to the structure, and the resonant frequency in the vertical direction is determined on the basis of a depositing, plating, or polishing process, by which the thickness of the structure is determined. Consequently, it is required to perfectly control the aforesaid processes, which is practically difficult. Furthermore, since the sensing electrodes are disposed at one side of the microgyroscope, the measurement is carried out nonlinearly when the large angular velocity is applied to increase the magnitude of the vibration in the vertical direction.
As described above, the conventional horizontal or tuning fork microgyroscope has problems in that vibration is transmitted to the structure of the microgyroscope due to external vibration or noise, and therefore other signals interference with the measurement of the angular velocity is considerably increased. In practical applications, occurrence of such interfering signals due to vibration or noise may have bad effects upon the performance of the microgyroscope.